Ecology: The depth zone separates crustaceans from deep-sea soft-bodied animals | Natural ecology and evolution

paper stating that distinct deep-sea transition zones at depths greater than 4 km separate deep-sea organisms by body type,nature and its evolutionMany animals with shells were found shallower than this transitional zone, while soft-shelled animals dominated at a deeper depth.

The deep sea zone is the most extensive habitat on Earth, covering more than 60% of the Earth’s surface, but it is also the least explored. At a depth of 3-6 km, no sunlight, and a temperature of 0.5-3°C, organisms must adapt to the extreme pressure. Although the ocean depths are thought to be less rich than the shallow water ecosystems, it is not clear how biodiversity changes within the deep sea zone.

Erik Simon-Lledó and colleagues collected images from 12 deep-sea expeditions in the Pacific Clarion-Clipperton Zone to analyze data on the presence of more than 50,000 megafauna (animals larger than 10 millimeters in size) living near the sea floor. As a result, two distinct regions are found in the deep-sea fauna. Soft corals, brittle stars (organisms related to starfish) and crustacean molluscs dominated the shallow-water communities at depths of 3.8–4.3 km, while sea anemones, glass sponges and sea cucumbers dominated the deep-sea communities at depths of 4.8–5.3 km. In the transitional zone between the two regions, these two communities were mixed. In the deep sea, biodiversity is maintained across these boundaries, although one might think that the deeper the depth, the lower the biodiversity. Simon-Lledó and colleagues suggest that such a distinct subdivision of the ocean depths may be determined by the depth of carbonate offsetting. The depth of carbonate compensation is the depth to which calcium carbonate in seawater becomes unsaturated, after which the conditions for animal shell formation become unsuitable.

Simon-Lledó and his colleagues argue that climate change and ocean acidification could alter this division and, combined with the effects of deep-sea mining, would weaken the ecosystem.

doi: 10.1038/s41559-023-02122-9

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